A. Field of the Invention
The present invention relates to a security feature having at least one luminescent substance for authenticating an object such as a security element, a security paper or a value document, and to security elements, security papers and value documents equipped with such a security feature, and to the use of the luminescent substance as a security feature substance for authenticating products of any kind.
B. Related Art
Security features or authentication features are marking means that allow a product such as for example a value document or a high-value commodity to be protected from forgery, or any forgeries to be distinguished from originals. Typically, security features have at least one feature substance, for example luminescent, magnetic or electrically conductive substances, which can be detected visually and/or by machine The feature substance or feature substances are arranged in a certain way, for example randomly or in the form of a geometrical or figured pattern or an encoding. This arrangement forms the so-called security feature. Security features for the purposes of the present invention have at least one luminescent substance as a feature substance.
A security element is understood to be an object such as a security thread, a label, a mottling fiber, a transfer element, etc., the object having at least one security feature and being adapted to be applied to an object to be protected, or incorporated into an object to be protected.
A security paper is understood to be a paper that is already equipped with at least one security feature or security element but is not yet fit for circulation, and is an intermediate product in the manufacture of a value document. A value document is understood to be the product fit for circulation.
Value documents are for example bank notes, checks, shares, value stamps, identity cards, credit cards, passports and other documents, as well as packages or other elements for product authentication.
The safeguarding of value documents against forgery by means of luminescent substances has been known for some time. For example, it is disclosed in EP 0 052 624 B2 to use luminescent substances on the basis of host lattices doped with rare earth metals.
In DE 10 2004 034 189 A1 there is described the use of rare earth and transition metal doped compounds with the formula XZO4 as a bank-note security feature, where X can be selected from the series of bivalent or trivalent cations, and Z can be, inter alia, niobium or tantalum.
It is also known to use luminescent substances with host lattices of alkali metal niobates and tantalates for applications in the areas of lasers, nonlinear optics, ferroelectrics, piezoelectrics. For these applications there are typically used single crystals pulled from the melt. However, crystals pulled from the melt do not have the ideal stoichiometric composition, but rather have lattice imperfections. In the case of lithium niobate, for example, pulling from a congruent melt results in a stoichiometry deviating slightly from LiNbO3, with a Li fraction of 48.4 atomic percent (based on Li+Nb). This causes vacancies to arise in the structure. The controlled incorporation of dopants is also difficult upon pulling from the melt, and the dopant concentrations in the melt and in the single crystal sometimes differ greatly.
From U.S. Pat. No. 6,330,939 B1 it is known to use LiNbO3 for specifically influencing the permittivity of a value document. However, LiNbO3 has no doping here and hence does not luminesce.
For luminescent security features there are preferably used substances in which either the absorption or the emission lies outside the visible spectral region. If the emissions lie at wavelengths between about 400 nm and about 700 nm, the luminescent substances are detectable with the eye upon suitable excitation. For some applications this is desired, for example in authenticity checking through illumination with UV light. However, for most applications it is advantageous when the emission lies outside the visible spectral region, because the security features can then be provided in hidden fashion. For detection, special detectors are required.
Luminescent feature substances for security features, i.e. luminescent substances with the characteristic properties that are suited for safeguarding value documents and in particular for automatic authenticity detection, are limited in number, however. Most inorganic and organic luminescent substances have uncharacteristic, wide spectra, a deficient emission intensity or other disadvantages, such as for example being difficult to manufacture. The use of commercially usual luminescent substances for security features is not very recommendable.
Starting out from this prior art, the present invention is based on the object of increasing the number of luminescent substances that are suited for manufacturing security features or authentication features, and in particular of providing security elements, security papers and value documents with security features that do not have the disadvantages of the prior art.
Essential properties that are aimed for in the present invention for the security-feature luminescent substances to be provided within the scope of this invention are in particular:    simple manufacturability with the defined, small grain sizes as are required for incorporation into or application to value documents;    high emission intensity, even at small grain sizes, and characteristic, i.e. well identifiable, emission spectra and/or absorption spectra; and    emissions preferably in the near infrared region.